How to Describe the Morphology of a Microscopic Gram of Stain
- 1). Determine whether the gram stain procedure was done correctly. View the microscopic slide under low power. Check that the background is clear, without coloring. This means that decolorization occurred as intended.
- 2). Determine whether gram-positive or gram-negative bacteria is present. Gram-positive bacteria will retain the crystal violet (purple) coloring during the staining procedure. Gram-negative bacteria retains the safranin red color. The difference is due to the high amount of lipids inside the cell walls of gram-negative bacteria. Alcohol added to the slide in the staining procedure draws out the lipid inside the cell wall, removing the crystal violet stain. The red coloring is retained.
- 3). Determine the shape of bacteria. Bacterial shape may be described as coccus, bacilli, vibrio, spirillium or spirochete. Spherical-shaped bacteria are cocci, rods are called bacilli, long rigid spirals are spirochetes, long flexible spirals are called spirillium. Bacteria taking on a short spiraled shape are referred to as vibrio.
- 4). Describe the arrangement of the bacteria. Cocci occur in pairs (diplococci) as with Streptococcus pneumoniae. Cocci can also appear as chains (streptococcus) or clustered like grapes (staphylococcus). Bacillus occurs as chains (streptobacillus) or individually (bacilli).
- 5). Determine the presence of endospores. Endospores, if present will appear inside rods (bacilli) as clear circles. Endospores will not stain due to their resistant outer coating. Generally endospores are produced by a limited number of genera such as Bacillus and Clostridium. Endospores are highly resistant, can survive radiation, high temperatures and without food. Spore-forming bacteria are used for biological warfare, according to Dr. Rao Sridhar, professor of microbiology. Bacillus anthracis, responsible for causing anthrax, is an example of a type of a spore-forming bacteria.
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